1. Field of the Invention
The present invention relates to systems for turbine inspection, and, more specifically to a turbine inspection device that permits non-destructive evaluation of a turbine.
2. Background Information
High availability and reliability of power generation systems has been a major requisite of the electric utility industry for many years. The high cost of unreliability and forced outages is well known, and industry experience has shown blade distress to be a leading cause of turbine-forced outages. Distress incidents are attributable to a variety of mechanisms, including cyclic fatigue and stress corrosion. Many of these incidents and related turbine damage could be eliminated if a reliable non-destructive inspection system could be developed. Early detection of blade distress is important in preventing and minimizing lengthy turbine forced outages.
Current field inspection of turbines typically requires extensive turbine disassembly and component cleaning prior to the performance of non-destructive inspection. A further problem exists in finding a reliable and inexpensive method of evaluating high stress areas on a turbine blade. Experience has shown that the areas in a turbine most susceptible to cracking due to, among other reasons, cyclic fatigue or stress corrosion, are the trailing edge of the blade airfoil, the blade root, and the rotor steeple area of the turbine (the area where the blade root engages with the turbine). It is known that the trailing edge, from the transition region near the top of the platform for any prescribed distance above the platform, is most prone to cracking. As for the turbine blade root and the turbine rotor steeple, the exit face surface area of those components is most prone to cracking. As is known to those skilled in the art, cracks are initiated and are visible on the turbine blade and turbine rotor steeple surfaces.
When turbine blades are removed for inspection, examination for cracks is done by fluorescent magnetic particle testing. This involves sandblasting the turbine blade to remove any scale build up which may have occurred during turbine operation and applying a magnetizing field to the blade with the aid of a magnetizing yoke coil or prod. Fluorescent magnetic particles suspended in a liquid vehicle are applied to the test surface. Cracks in the blade disrupt the magnetic field creating a leakage site which attracts the tiny suspended particles. A "black" light is used to facilitate the inspection. Crack lengths can then be measured and compared to calculated allowable critical crack lengths. Depending on the results from the fluorescent magnetic particle test, the blades may then be replaced as necessary.
This method of turbine inspection, where turbine blades are removed from the turbine rotor steeple, is both time consuming and expensive. Also, good blades may possibly be damaged when being removed or inserted into the turbine. Damage to rotor steeples is also possible during blade removal.
It is also known to inspect a turbine by placing a piece of conduit, with an eddy current coil sensor and/or a small television camera fastened to one end of the conduit, inside a turbine. The operator manually probes the turbine test areas, using the camera to help place the eddy current coil on the area to be inspected. Problems with this type of device include inaccurate positioning of the eddy current coil on the area to be inspected and failure to adequately inspect all critical areas where cracks may be present within the turbine. This can lead to missed or future calls resulting in damage to the turbine and replacement of the turbine blades.
Accordingly, there exists a need for a turbine blade inspection system which permits non-destructive inspection of turbine blades and turbine rotor steeples while overcoming the disadvantages of presently used methods. Further, the system should provide for rapid assessment of blade and rotor steeple metal integrity during a brief unit shutdown and in highly confined spaces, be inexpensive and simple to use, and pose minimal safety and health risks.